Memory devices provide data storage for electronic systems. One type of memory is a nonvolatile memory known as flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that may be erased and reprogrammed in blocks. Many modern personal computers have BIOS stored on a flash memory chip. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized, and to provide the ability to remotely upgrade the devices for enhanced features.
NAND is a basic architecture of flash memory. A NAND cell unit comprises at least one select gate coupled in series to a serial combination of memory cells (with the serial combination being commonly referred to as a NAND string).
A typical flash memory comprises a memory array that includes a large number of nonvolatile memory cells arranged in row and column fashion. The cells are usually grouped into blocks. Each of the cells within a block may be electrically programmed by charging a charge-retaining material (for instance, polysilicon of a floating gate, or charge trapping material, such as silicon nitride and/or nanodots). The charge may be removed from the charge-retaining material by a block erase operation. Data is stored in a cell as charge in the charge-retaining material.
Individual memory cells may comprise charge-blocking dielectric material over the charge-retaining material, and a control gate material over the charge-blocking dielectric material. The charge-blocking dielectric material may comprise crystalline material having a dielectric constant greater than or equal to about 15. Materials with dielectric constants greater than or equal to 15 may be referred to as ultra-high k dielectric materials.
A problem with conventional memory cells can be that the crystalline ultra-high k dielectric materials utilized as charge-blocking materials may have grain boundaries extending therein. Such grain boundaries may enable contaminating materials to leak into, or even entirely through, the charge-blocking materials. Also, grain boundaries may contain a high density of defects which may provide leakage paths, and/or may provide unwanted trapped charge.
It is desired to develop new memory cells which avoid the above-discussed problem, and to develop methods of forming such memory cells.